Generally, a semiconductor light-emitting device comprises a first conductive clad layer, an active layer, and a second conductive clad layer sequentially formed on a substrate. When a voltage is applied to the light-emitting device through electrodes connected to the first and second conductive clad layers, respectively, the semiconductor light-emitting device allows electrons and holes to be recombined in the active layer thereof, and thus generates light within a specific frequency range.
Light-emitting efficiency of the semiconductor light-emitting device depends on internal quantum efficiency and light extraction efficiency (which can also be referred to as external quantum efficiency). Particularly, the light extraction efficiency depends on optical factors, that is, the refractivity of respective layer structures and/or the flatness of an interface between the layers.
In view of light extraction efficiency, the semiconductor light-emitting device has fundamental limitations. That is, larger refractivity of the semiconductor layers constituting the semiconductor light-emitting device compared with the atmosphere or the substrate decreases a critical angle, which determines an incident angle of light to be emitted, and results in the total internal reflection of significant amounts of light generated from the active layer, whereby the light travels substantially in a lateral direction, causing a significant amount of the light to be lost inside the light-emitting device or to be emitted in the undesired lateral direction.
For instance, in a nitride semiconductor light-emitting device, since GaN has a refractivity of 2.4, an incident angle larger than 23.6 degree., that is, a critical angle at the GaN/atmosphere interface, results in the total internal reflection of the light generated from the active layer, thereby forcing the light to travel within the light-emitting device in the lateral direction, so that the light is lost inside the light-emitting device or emitted in the undesired lateral direction. Accordingly, the light extraction efficiency is merely 6% at the GaN/atmosphere interface. Moreover, since a sapphire substrate has a refractivity of 1.78 similar to that of the GaN layer, the light extraction efficiency is merely 13% at a GaN/sapphire substrate interface.
In order to solve such a problem, US Patent Publication No. 2003-57444 (Publication date: Mar. 27, 2003, Assignee: Nichia Corporation) discloses a semiconductor light-emitting device in which an irregular portion 13 comprising protrusions or depressions is formed on a substrate 11 of the light-emitting device, as shown in FIG. 1a. According to the disclosure, the irregular portion 13 on the substrate 11 can substantially increase an incident angle of a GaN/sapphire substrate interface, thereby enhancing the light extraction efficiency of the GaN/sapphire substrate interface.
The above-mentioned semiconductor light-emitting device can enhance the light-emitting efficiency of the device to a certain level (light-emitting efficiency enhanced about 20%) by enhancing the light extraction efficiency through the irregular portion on the substrate. However, in view of a chip scale light-emitting device, since there is no consideration of an appropriate position of the irregular portion, the semiconductor light-emitting device of the disclosure cannot completely overcome the problem of optical loss due to the light emitted through side surfaces of the device. More specifically, the disclosure did not suggest a solution for extracting the light, which is generated from an active region of side surfaces of the device or reaches the side surfaces through total internal reflection, in a desired direction.
Referring to FIG. 1b, the construction of the chip scale light-emitting device is schematically illustrated which has the irregular portion 13 of FIG. 1a on the substrate 11. The chip scale light-emitting diode has a first conductive clad layer 14, an active layer 15, and a second conductive clad layer 16 sequentially formed on the substrate 11. As shown in FIG. 1b, since the irregular portion 13 is not formed on the side surfaces of the substrate 11, that is, along edges of an upper surface of the substrate 11, a significant amount of light, which is generated from the active region of the side surfaces of the device or reaches the side surfaces through the total internal reflection, is lost or cannot be extracted from the device in the vertical direction. As a result, the above described semiconductor light-emitting device has limitation in enhancing the light extraction efficiency.
As such, what is needed is to provide a technology for enhancing the light extraction efficiency in view of the chip scale light-emitting device, in order to improve the light-emitting efficiency thereof.